Safe method for testing the electrically conductive firing element of an explosive device by rotating and counterrotating a portable galvanometer



Oct. 20, 1964 M. w. BOLTON 3,153,757

SAFE METHOD FOR TESTING THE ELECTRICALLY CONDUCTIVE FIRING ELEMENT OF AN EXPLOSIVE DEVICE BY ROTATING AND COUNTERROTATING A PORTABLE GALVANOMETER Filed May 15 1961 INVENTOR. MARQUIS W. B LTON BY WM},

ATTOR Y E s/ %WZZ AGENT United States Patent 3,153,757 SAFE METHOD FOR TESTING THE ELECTRI- CALLY CONDUCTIVE FIRING ELEMENT OF AN EXPLOSIVE DEVICE BY ROTATING AND COUN- TERROTATING A PORTABLE GALVANOMETER Marquis W. Bolton, Burbank, Calif, assignor to the United States of America as represented by the Secretary of the Air Force Filed May 15, 1961, Ser. No. 110,275 1 Claim. (U. 324-51) The purpose of this invention is to provide a safe and reliable method for checking squibs or other explosive devices detonated by sending an electrical current through a resistance element embedded in an explosive material.

Such devices are widely used in missiles as explosive bolts, spin rockets, igniters for solid fuel rocket motors, etc., and their proper operation in the launching and flight program of the missile is of vital importance. A reliable method for checking these devices is therefore necessary. For maximum reliability, checking should be accomplished with explosive devices in place in the loaded missile and as short a time as possible before the launching of the missile. When checking is done on a loaded missile and particularly when igniters for solid fuel rocket motors are involved, it is apparent that the risks are very great and there is a notable reluctance on the part of persons responsible for safety to allow tests which could conceivably cause an inadvertent ignition of a rocket motor.

Methods for checking electrical explosive components depend on providing suflicient current flow to establish the fact of electrical continuity without at the same time providing enough current to detonate the explosive device. Presently available testers utilize a current flow of several milliamperes. In the proposed method the current flow produced is about two microamperes. The advantage is apparent in that danger of accidental detonation is reduced by a factor of 1000.

Briefly, the method consists in connecting the explosive device to be tested across the terminals of a sensitive portable galvanometer, alternately rotating and counterrotating the galvanometer about the axis of rotation of the galvanometer needle and observing the amplitude of the needle oscillation about its zero point. If the explosive device is defective, i.e., does not have electrical continuity, its presence in the galvanometer circuit has no damping effect on the needle oscillations and they are therefore of large amplitude. On the other hand, if the explosive device is good, the damping produced by the relatively low resistance of its firing element reduces the needle oscillation to a low amplitude. The two conditions are easily distinguishable, and since the only current flow through the resistive element of the explosive device is that due to the minute voltage induced in the galvanometer coil by its slight movement in the magnetic field of the meter, there is no danger of detonation.

The invention will be described in more detail with reference to the accompanying drawing in which- FIG. 1 shows schematically the electrical details of a galvanometer of the type used in the described testing method, and

FIGS. 2 and 3 illustrate the manner of carrying out the test method and the indications obtained.

Referring to FIG. 1 and reviewing the construction of a galvanometer of the rotating coil type, the instrument contains a coil 1 positioned for rotation in circular air gaps formed by the cylindrical faces 2 and 3 of permanent magnet 4 and core 5. A uniform magnetic field exists in the air gaps with the flux lines normal to the pole faces and the surface of core 5. The coil 1 is attached to and supported by shaft 6 which extends through a central passageway in core 5 and is freely pivoted at 3,153,757 Patented Oct. 20, 1964 "ice its ends. Shaft 6 also carries the needle or pointer 7 of the instrument which has a balancing counterweight 8. Spiral spring 9 and a similar spiral spring (not shown) at the other end of coil 1 serve the dual functions of providing a restoring force for the needle and providing electrical connections to the ends of the coil. Electrical leads 11 and 12 connect the spiral connectors to the galvanometer terminals 13 and 14.

Galvanometers are normally used as current measuring devices. When a current flows in coil 1 a force is exerted on the conductors of the coil located in the magnetic field of the air gap which rotates the coil and needle 7 away from the zero position through an angle proportional to the magnitude of the current and in a direction determined by the direction of current flow.

In the proposed testing method a galvanometer of the above type is used in a different manner. It is apparent that if coil 1 is mechanically rotated back and forth through its zero position, in which position no flux links the turns of the coil, an alternating voltage will be induced in the coil by the magnetic field linking the coil first in one direction and then in the other. No current will flow in coil 1 as a result of the generated voltage if the external circuit between terminals 13 and 14 is open. However if a conductive path is provided between the meter terminals the generated voltage will cause a current to flow in the coil and the effect of this current will be to cause a force to be exerted against the coil that opposes its motion in the magnetic field. Hence a low resistance external path between terminals 13 and 14 will exert a marked damping effect on the oscillation of coil 1 about its neutral position.

The proposed method for testing electrical explosive devices utilizes the above principle and consists in connecting the explosive device across the terminals of a galvanometer, introducing mechanically an oscillation of the galvanometer needle about its zero point and observing the amplitude of the needle oscillation. The procedure is illustrated in FIGS. 2 and 3. The terminals 13 and 14 of a portable galvanometer 15, of the type shown in FIG. 1, are connected to the terminals 16 and 17 of an electrical explosive device by means of flexible leads 18 and 19. The resistance 20 represents the conductive firing element of the explosive device. The oscillation of the needle about its zero position may be induced by alternately rotating and counterrotating the body of the galvanometer about the rotational axis of the needle.

Considering further the matter of needle oscillation, when the instrument is rotated and counterrotated the tendency is for the needle to remain stationary, because of the inertia of the needle-coil assembly, and for the meter scale to rotate relative to the needle. When the external circuit of the galvanometer is open, as is the case in FIG. 2 Where the device being tested has an open resistance element 20, the only rotational forces transmitted to the needle are those due to needle pivot friction and the spiral springs 9. Since these forces are very small in a sensitive galvanometer, the effective oscillation of the needle about zero on the meter scale under open circuit conditions is large, as illustrated in FIG. 2. In the case of a good explosive device, however, the relatively low resistance element 20 establishes electrical continuity in the meter coil circuit so that the voltage induced in the coil by its movement in the magnetic field of the meter causes a current to flow in the coil which in turn results in a force on the coil opposing its movement in the field as stated before. Therefore, under the closed circuit condition, the rotational forces applied to the needle-coil assembly by pivot friction and the spiral springs are greatly augmented by the force due to coil current. This causes the needle to follow the movements a; of the meter body much more closely or, in other words, causes a marked decrease in the amplitude of the eflective oscillation of the needle about zero on the meter scale, as illustrated in FIG. 3. It will be noted that, during rotation and counterrotation, when the movement of the needle relative to zero on the scale is large its movement in space is small and vice versa.

The highly damped oscillations of FIG 3 are easily distinguished from the relatively free oscillations in the open circuit case of FIG. 2, and since the current flowing in element 2% is only that due to the small voltage induced in the meter coil by its relatively slight movement in the magnetic field of the meter, specifically about 2 microamperes, there is no danger at all of an inadvertent detonation of the explosive device being tested.

I claim:

The method for testing the electrically conductive firing element of an explosive device using a portable galvanometer having a rotating coil and indicating needle assembly With its zero position at the center of the galvanometer scale, said method comprising the steps of connecting said firing element in a closed loop consisting of said element and said galvanometer coil in series, alternately rotating and counterrotating said galvanometer about the rotational axis of said coil and needle assembly, and observing the amplitude of the oscillation of the needle about zero on the galvanometer scale, a large amplitude relatively free oscillation indicating a defective firing element and a small amplitude highly damped oscillation indicating a good firing element.

References Cited in the file of this patent UNITED STATES PATENTS Hofbauer Aug. 1, 1893 OTHER REFERENCES 

